In the commercial manufacture of benzene di- or tricarboxylic acids (e.g., isophthalic acid [IPA], terephthalic acid [TA] or trimellitic acid [TMLA]), a residue is obtained (after maximizing recovery of such acid and recovery for reuse of the reaction solvent), which is a mixture of oxygen-containing derivatives of benzene and toluene which are mono-, di- and tricarboxylic acids, aldehydocarboxylic acids, and methylol-substituted benzene or toluene or their carboxylic (benzoic or toluic) acids and which also contains components of catalysis. Usually such components of catalysis are Co-Mn-Br, Co-Zr-Mn-Br or Co-Zr-Mn-Ce-Br from liquid-phase oxidation of a xylene or pseudocumene (1,2,4-trimethylbenzene) with air in the presence of acetic acid reaction solvent. A similar residue is also obtained from the neat oxidation of liquid o-xylene with air in the presence of Co-Mn-Br catalyst system after dehydrating the o-phthalic acid formed to its anhydride under conditions which vaporize the anhydride, water and materials boiling between the anhydride and water. While such residues amount to from 2 to 25 weight percent of the benzene di- or tricarboxylic acid produced, such residue production annually is substantial in view of the millions of kilograms of the benzene di- or tricarboxylic acids produced annually.
Such residues contain water-soluble benzene carboxylic acids and water-soluble forms of the components of catalysis. Landfill disposal of such residues is undesirable because rain and groundwater leach out those carboxylic acids and soluble forms of the components of catalysis and can contaminate surface run-off water and eventually streams as well as below-surface aquifers. Disposal of the organic residues an be made by the processes as disclosed in U.S. Pat. Nos. 4,258,227, 4,266,084 and 4,393,264 which are incorporated into this application and made part hereof. The catalyst components in the aforementioned U.S. patents are converted to forms in the resultant ash which are difficult and/or expensive to convert to reusable forms for the oxidation of the methyl-substituted benzenes. Although, in such residues, the substituted benzene and toluene compounds, whose substiuents are the carboxy-, aldehyde- and methylol substituents, are individually desirable and useful commercial products, it is not economically feasible to separate and recover the individual compounds from the residues.
Cobalt and manganese acetates are major components of the catalyst system used for oxidizing p-xylene, m-xylene and pseudocumene. These catalyst metals are present in the oxidation residues produced by the various oxidation processes. The recovery of cobalt and manganese from the oxidation residues would not only reduce catalyst costs but also reduce disposal problems associated with disposal of the oxidation residues.
As an example, trimellitic anhydride (TMA) can be prepared by oxidation of pseudocumene in the presence of a catalyst comprising cobalt, manganese and bromine. Cobalt is an especially valuable metal and its cost is a substantial portion of the cost of the oxidation process. Fresh catalyst is necessary in the process because the oxidation is product-inhibited and difficult to complete. Mother liquor recycle containing catalyst components is not feasible because many by-product compounds which would act as inhibitors of the process would also be recycled. Although the catalyst components in the oxidation residue can be water-extracted, aqueous extraction cannot be used because trimellitic acid is very soluble in water.
Oxidation residue from p-xylene, m-xylene or pseudocumene oxidations containing cobalt and manganese catalyst metals can be disposed of by incineration or by other processes disclosed in the aforementioned three U.S. patents. A complex incineration residue of mixed metal oxide ash of cobalt and manganese is produced by the incineration of this oxidation residue. The mixed metal oxide ash from incineration is recovered in the form of clinkers and fly ash. The fly ash is collected in the electrostatic precipitator or bag filler of the residue incinerator. A cobalt and manganese aliphatic carboxylic acid solution can be obtained directly from this complex incinerator residue by contacting it with the aliphatic acid at elevated temperatures and pressures. Other processes use strong acids, such as hydrochloric acid, hydrobromic acid, sulfuric acid or nitric acid, to recover the metals from the complex incinerator residue but further processing is required to obtain cobalt and manganese acetate from these strong acid solutions.
Methods of recovering cobalt compounds are known. For example, G.B. Pat. No. 262,075 and U.S. Pat. No. 1,637,281 teach the recovery of cobalt as cobaltous acetate from cobaltic hydroxide, obtained during refining of cobalt from cobalt-containing ores, by heating cobaltic hydroxide with acetic acid in a closed vessel and in the presence of a reducing agent to a temperature above the boiling point of the acetic acid. Preferably the reducing agent is metallic cobalt in the form of a fine powder. The increasing pressure caused by use of the closed vessel causes the hydroxide to solubilize rapidly at a pressure of about 2.5 to 3.25 atmospheres. The amount of cobalt metal added forms cobaltous oxide from the cobaltic hydroxide and is approximately that which is theoretically required to convert all the cobalt to cobaltous acetate. The products of the reaction accordingly are a mixture of cobaltous oxide and cobaltous acetate.
Several methods are known for recovering cobalt-manganese oxidation catalysts. U.S. Pat. No. 2,964,559 teaches the recovery and recycle of oxidation catalysts employed for the liquid-phase oxidation catalysts employed for the liquid-phase oxidation with molecular oxygen of aliphatic substituted aromatic compounds to aromatic acids by extraction of the heavy metal catalyst from the bottoms fraction remaining after distillation of the reaction mixture. Extraction is with water or a lower-saturated monocarboxylic acid. The extract can then be recycled. Nickel, iron and chromium, present as a result of corrosion, are also extracted. However, as is taught in U.S. Pat. No. 3,341,470, the presence of iron, chromium and copper, among other heavy metals, are antagonistic when present as minor contaminants in oxidation processes using the combination of cobalt and manganese. U.S. Pat. 3,341,470 teaches the cobalt-manganese oxidation catalyst can be recovered in a contaminant free form by dissolving the incinerated oxides in sulfuric acid containing chlorides to reduce the manganese to a soluble divalent form, followed by neutralization with calcium hydroxide and buffering with calcium carbonate to precipitate iron and chromium oxides. The cobalt and manganese are precipitated as carbonates by addition of sodium carbonate and then treated with acetic acid to obtain the desired acetate. Copper can be removed with a soluble sulfide, as hydrogen sulfide. U.S. Pat. No. 4,417,972 teaches the recovery of metal catalysts from carbonaceous material and ash by oxidation in a combustion zone to oxides. The oxidized solids are then treated with an aqueous solution of a basic metal salt to extract the oxides as soluble alkali metal salts which can then be recycled to the reaction. U.S. Pat. No. 4,546,202 teaches a process for recovery of cobalt and manganese from solid aromatic acid oxidation incinerator ash, pyrolysis sand or pyrolysis char by heating the residue at a temperature of about 120.degree. C. to about 300.degree. C. and a pressure of about 3 atmospheres to about 30 atmospheres in the presence of acetic acid. Water can be added after the reaction to solubilize all the recoverable metal acetates. Extraction of incinerator ash comprising fly ash was less efficient because the fly ash contained oxides which were more difficult to extract.
Accordingly, a number of processes are known for recovery of metal catalysts from oxidation residues. However, known processes require process conditions which necessitate corrosion-resistant equipment, or pressure equipment, or combinations of corrosion-resistant equipment with high pressure equipment and high temperatures and additional processing after recovery of the metal components in a form suitable for recycling to the oxidation reaction. For example, strong mineral acids, such as hydrochloric acid, hydrobromic acid, sulfuric acid and nitric acid, which will react with the metal in the ash, are used. Depending on the acid used, halogens, SO.sub.x or NO.sub.x are released into the vent gas and scrubbers must be employed to remove these materials from this stream. Also, special equipment, such as glass-lined vessels and piping, is required to handle these strong mineral acids. Once the cobalt and manganese are reacted, further processing must take place to convert the cobalt and manganese ions into the acetate salts.
In our novel process, acetic acid at about the boiling point or near reflux temperature (108.degree. C.) and atmospheric pressure is used to extract the cobalt and manganese from incinerator ash comprising fly ash and clinkers by solubilizing the cobalt and manganese directly in the form of the acetate salts. Special glass-lined or titanium equipment is not required to handle this processing, and since only water and nitrogen are produced as by-products, special scrubbers are not required.
Accordingly, our novel process avoids not only high pressure titanium equipment but permits the use of cobalt metal or other suitable reducing agents, such as hydrazine, which are particularly economical or particularly effective in recovery of the catalyst metals.